This invention relates to fuel injection devices for spark ignition type internal combsution engines, and more particularly to control devices which detects the transient states of the engine and controls, in accordance with the detected transient state, the amount of fuel to be injected into the air inlet passage to the cylinders of internal combustion engines.
Conventionally, the supply of fuel to the spark ignition type internal combustion engines of passenger automobiles has been effected by carburetors; recently, however, fuel injectors are becoming increasingly common. These fuel injectors are capable of supplying a precisely controlled amount of fuel to the internal combustion engine so as to obtain an optimum air-fuel ratio. However, the supply of fuel to the combustion rooms in the cylinders of an engine may be delayed when the amount of air intake varies during the transient state, such as the slow accelerating transient state, of the engine; this delay of the fuel supply may be caused, for example, by: the delay in the detection of the amount of air intake; the delay in the calculation of the fuel amount; and the time lag resulting from the length of time which the fuel injected into the air intake pipes takes before it reaches the cumbustion rooms in the cylinders of the engine.
Thus, the correction of the fuel amount during the transient state of the engine is effected, for example, in this manner: the intake air pressure, which corresponds to the operating state of the engine, is detected, and the variation or increment the air pressure over a length of time is compared with a threshold level, to determine whether the engine is in the transient state or not; upon detection of the transient state of the engine, the amount of fuel is adjusted in accordance therewith.
However, the detection signal of the intake air pressure containns ripples, i.e., small fluctuations resulting from the pulsations within the air inlet passage to the cylinders of the engine; in particular, in the case of the engines provided with a supercharger, such as a turbocharger, the ripples in the air pressure detection signal become extremely consipicuous in the supercharged region: FIG. 1 shows a typical variation curve of the intake air pressure of an engine provided with a supercharger, wherein the air pressure rises rapidly (with small ripples) to the atmospheric pressure P.sub.10, and, thereafter, in the supercharged region, varies slowly, with, however, manifest ripples of large amplitudes.
For the purpose of attenuating these ripples and reducing the adverse effects thereof to the detection of the transient state of the engine, the fuel injection control device generally comprises an analog filter circuit: the detection signal of the intake air pressure is passed through the filter circuit to attenuate the ripples contained therein. However, the provision of an analog filter circuit is not sufficient. Namely, if the attenuation factor of the ripples through the analog filter circuit is increased, the delay in the detection of the transience of the egnine becomes manifest, with the result that the supply of fuel to the engine is delayed, and, as a result, the air-fuel ratio is deviated from its optimum level. On the other hand, if the ripples contained in the air pressure detection signal is not sufficiently attenuated by the analog filter circuit, the fluctuations of the increment resulting from the ripples may cause erroneous detection of the transience of the engine.